1. Field of the Inventions
The field of the invention relates generally to systems and methods for blocking the propagation of electromagnetic waves in parallel plate structures and more particularly, to electromagnetic noise reduction in layered electronic devices.
2. Background Information
Electronic devices are often configured as layered assemblies of two-dimensional structures. These layered assemblies, which include such familiar devices as printed wiring boards, multi-chip modules, and integrated semiconductor chips, are known generally as “multi-layer preformed panel circuits” or simply “panel circuits”. Electromagnetic noise propagation in panel circuits has become increasingly problematic as increases in performance dictate simultaneously higher device operating speeds and decreased operating voltages. Both of these trends have converged to make these devices more susceptible to electronic noise, thereby limiting panel circuit performance because of spurious electrical signal levels generated primarily within the panel circuit itself. One particularly troublesome noise source is high-speed digital switching noise imposed on the nominally constant voltage (DC) power distribution system. DC power distribution is most commonly accomplished in panel circuits by means of two closely spaced and substantially parallel conductors, generally referred to as the “power plane” and the “ground plane”. This general structure unfortunately also enables radio frequency (RF) noise propagation throughout the device by acting as a parallel-plate waveguide (PPW). Various means have been employed to attenuate this electronic noise by, for example, inserting strategically placed shunt capacitors and/or selecting the location of sensitive components to correspond to voltage minima in the noise spatial distribution. These methods are relatively effective at frequencies below about 500 MHz. Above this frequency level, there remains a growing need for more effective means of electrical noise isolation.
One possible approach to mitigating the effect of power plane noise in panel devices would be to impose within the power plane, RF blocking filter structures that could operate effectively above 500 MHz. A particularly effective design of high-frequency RF filter is the ‘waffle iron’ filter, first proposed in 1950 by S. B. Cohn. Waffle iron filters are a type of low-pass corrugated waveguide filter that contains both transverse and longitudinal slots cut into the internal walls of the rectangular guide. These slots create opposing teeth or bosses, resulting in the structure suggesting its name. Waffle iron waveguide filters, as depicted generically in FIG. 1, are characterized by wide, high-attenuation, two-dimensional “stop bands”. Stop bands are spectral regions where electromagnetic wave propagation is impeded due to the constraints imposed by a periodic array of interactive elements. The topology, capacitance, and inductance of the waffle iron filter's resonant elements define its stop band characteristics. Waffle iron filters were originally designed for high power microwave applications, and its embodiments for this application are not well suited to small-scale panel circuits. However, the technologies that have arisen in support of circuit board, integrated circuit, and other types of panel circuit fabrication are adaptable to making various types of miniaturized resonant elements that could be configured to produce compact filters that have a similar effect, or characteristics of a waffle iron type of filter.
For example, in printed wiring boards (PWB's), “resonant vias” can be fabricated. A resonant via is used here to denote a shunt electrical circuit containing one or more plated through holes (PTH) in series with one or more capacitors. PTH's are routinely fabricated in PWB'S, as well as in other panel circuit devices such as multi-chip modules and integrated ciruits (IC's). The term “resonant via” was introduced by Sedki Riad in his U.S. Pat. No. 5,886,597 (Riad). However, Riad more narrowly claimed a resonant via as one PTH in series with one capacitor and his patent emphasizes only the RF decoupling application for resonant vias, where they are used as a low impedance interlayer connection between metal layers in multilayer PWB's. Although Riad discloses the use of multiple resonant vias, he does not suggest the idea of employing a periodic array for the purpose of creating a well defined stop band filter.